The function of the TOR (target of rapamycin) protein kinase pathway is central to cell growth control in eukaryotes. Rapamycin and its analogs are under intense investigation for their potential in cancer therapies. The broad, long-term goals of this proposal are: (I) To better understand the molecular mechanisms of TOR signaling in cell growth control, (II) To elucidate the mechanisms of cellular sensitivity to rapamycin, and (III) To learn how TOR signaling properties can be exploited to advance the treatment for human diseases linked to this pathway. Towards these ends, we are developing a novel chemical-genetic and proteomic approach to rapidly and efficiently discover using the budding yeast, Saccharomyces cerevisiae, which is an established model to study TOR signaling, cell growth control, and cancer-related, conserved pathways. In preliminary studies, we have discovered two classes of novel small molecules that modify rapamycin's growth-inhibitory effects: the SMIRs (small molecule inhibitors of rapamycin) and the SMERs (small molecule enhancers of rapamycin). Our studies of the SMIRs have led to the discovery of new mechanisms in regulating TOR pathway activity. We will apply similar concepts and methods to study the SMERs, which we envision to be useful for a) enhancing rapamycin's therapeutic efficacy, b) treating rapamycin-insensitive tumors, and/or c) preventing the development of drug resistance to rapamycin, all of which are expected to have significant impact on TOR-targeted therapies. The hypothesis that will be tested is that perturbations of the TOR pathway create a set of distinct cellular states that can be selectively targeted to cause reduction (or loss) in fitness in cancer cells. Our specific Aims are: 1) to perform high- throughput screening to identify small molecules that are synthetic-lethal with perturbations of the TOR pathway, 2) to characterize the SMERs and test their activities in mammalian cells, and 3) to identify the protein targets for the SMERs and elucidate their mechanisms in TOR pathway modulation. Upon successful completion, the project will have demonstrated a new paradigm for cancer drug discovery, target identification, and mechanism studies in search of cures, which aligns closely with the missions of the NCI/NIH. The search for effective therapies for cancer presents a compelling challenge to the scientific and medical community. Our studies aim to combine two of the most powerful concepts in this area, namely molecularly targeted therapy and synthetic lethality, to achieve greatest specificity, efficacy, and safety in therapeutic intervention. We believe that our studies will shed new light on basic mechanisms of TOR signaling as well as TOR-targeted therapy. Furthermore, the methods that we develop and test here will be broadly applicable to the study of other cancer drugs, other pathways and other human diseases.